A radioactive source is used to monitor the rate of decay over a period of time. The measurement device records a total count rate that includes a constant background radiation of 15 counts/minute15\text{ counts/minute}15 counts/minute.
If a graph is plotted with the count rate on the vertical axis and time on the horizontal axis, which of the following describes how the curve of the corrected count rate compares to the curve of the measured total count rate?
The corrected curve is shifted vertically upwards by 15 counts/minute15\text{ counts/minute}15 counts/minute at all times.
The corrected curve is shifted vertically downwards by 15 counts/minute15\text{ counts/minute}15 counts/minute at all times.
The corrected curve is shifted horizontally to the left by 15 minutes15\text{ minutes}15 minutes at all points.
The corrected curve is shifted vertically downwards by a variable amount that decreases to zero as the source decays.
484 exam-style questions on CIE IGCSE Physics 5.2 Radioactivity, covering 5.2.1 Background radiation and measuring count rate, 5.2.2a Alpha, beta and gamma emission, 5.2.2b Deflection and ionising effects of nuclear radiation, 5.2.3a Radioactive decay, 5.2.3b Effects of decay on the nucleus and decay equations, 5.2.4a Half-life, 5.2.4b Choosing isotopes for applications, and 5.2.5 Effects of ionising radiation and safety precautions. Each one has a worked solution and a mark scheme showing where the marks go.